A conventional boiling water reactor includes a reactor pressure vessel within which is disposed a nuclear reactor core surrounded by an annular core shroud. The core shroud is spaced radially inwardly from the reactor vessel to define an annular flow channel, or downcomer, in which is recirculated reactor coolant water. The coolant water typically flows downwardly from near the top of the reactor vessel, through the downcomer, around the bottom of the core shroud and then upwardly through the reactor core wherein it is heated for generating steam which is then suitably channeled to a conventional steam turbine which powers an electrical generator for generating power for a utility grid.
The steam turbine extracts energy from the steam and causes the steam to condense into its liquid phase for forming feedwater. The feedwater is then returned to the reactor vessel under pressure from a conventional feedwater pump in a basically closed cycle. The feedwater returned to the reactor vessel is discharged from a conventional feedwater sparger from which it mixes with the reactor coolant water therein for repeating the cycle.
In order to increase the recirculation of the coolant water in the reactor vessel, conventional coolant pumps are provided, in one example, external of the reactor vessel and are suitably connected thereto by external piping. This is conventionally done to increase the reliability of the reactor for preventing damage to internal reactor components due to any failure of the coolant pumps. The external piping loop system associated with the use of external coolant pumps has increased maintenance requirements in view of the relative complexity thereof and may lead to substantial radiation exposure to maintenance personnel working adjacent thereto due to radioactive corrosion deposition within the pipes.
In one advanced boiling water reactor design, a coolant pump is located inside the reactor vessel and is powered by a motor located outside the reactor vessel. Accordingly, a rotating shaft must extend through the reactor vessel and must include suitable seals for preventing leakage of the high pressure coolant water which flows within the reactor vessel. Such pump and motor arrangement is relatively complex and requires a relatively complex shaft seal for preventing leakage. Furthermore, a typical boiling water reactor allows placement of the pump and motor arrangement in a limited area typically at the bottom of the reactor vessel adjacent to conventional control rod drives thus increasing congestion of these components and increasing the complexity o maintenance thereof.
Conventional motors for powering the coolant water pumps are typically limited in their ability to withstand elevated temperature and elevated pressures associated with a conventional boiling water reactor. For example, the coolant water channeled within an exemplary reactor vessel has a temperature of about 520.degree. F. (271.degree. C.) for an exemplary embodiment, and has a pressure of about 1,000 psi (6.89 MPa). Such a high temperature would severely shorten the service life of conventional motors since the materials thereof will rapidly degrade at such elevated temperatures. Furthermore, the high pressure of the coolant water would have to be accommodated in suitable seals associated with the motor. Furthermore, the environment inside the reactor vessel includes radiation which is known to degrade conventional electrical insulation used in electric motors as well as degrade conventional hydrocarbon-type lubricants also used therein. Such lubricants also present the additional problem of contamination of the reactor coolant water if they should leak from the motor or pump.
Accordingly, electrical motors typically used for driving pumps are located external of the pressure vessel to avoid degradation due to radiation or high temperature and to eliminate the possibility of contamination of the coolant water. However, where the motor is located external to the reactor vessel and the pump is located internal to the reactor vessel, suitable high pressure seals must still necessarily be provided for containing the high pressure coolant water within the reactor vessel without leakage.